1. Field of the Invention
The present invention generally relates to electronic circuits, and more specifically to common-mode filters used in such circuits.
2. Discussion of the Related Art
FIG. 1 is a block diagram of an electronic board 1 comprising, in addition to one or several circuits 12 capable of providing differential signals, at least one common-mode filter 2 (CMF) of the type to which the present invention applies. Such a filter comprises two input terminals 22 and 24 and two output terminals 26 and 28. The input terminals are intended to receive the differential signals to be processed, the filtered result of which is output.
As illustrated by the examples of signal curves in FIG. 1, the function of a common-mode filter is to let through the differential signals which are in phase opposition and to attenuate or cut the common-mode signals and noise which are in phase on the two inputs. An ideal common-mode filter is a short-circuit for signals in phase opposition at the input and an open circuit for signals in phase. In practice, the filter always lets through low frequencies, and thus a bias level common to the two differential signals.
Common-mode filters are generally used for differential data accesses in digital processing circuits, especially if the frequency of the useful signals is in the same range as the frequency of the common-mode noise or signals which are desired to be rejected.
Common-mode filters can also be found in analog circuits, for example, to process signals in differential microphones, where the signal is transmitted symmetrically.
FIG. 2 shows an example of a usual common-mode filter 3. Such a filter is formed of two inductances 31 and 33 respectively connecting input and output accesses 32 and 36 and input and output accesses 34 and 38. Inductances 31 and 33 are coupled by a magnetic material 35. The respective phase points of inductances 31 and 33 are on the same side (in the example, on the side of input terminals 32 and 34) to avoid filtering signals in phase opposition. The operating principle of filter 3 of FIG. 2 is to exhibit a high series impedance for signals in phase and a low series impedance for signals in phase opposition.
The use of a magnetic material 35 is linked to the need to have high inductances having a high series impedance in common mode, without the size of the filter becoming too large.
This solution is thus limited to applications in which such a magnetic material can be used. In particular, this may not be possible when common-mode filters are desired to be integrated, for example on semiconductor or isolating substrates (for example, glass). Integrating inductive elements with the deposition of a ferromagnetic layer is not only difficult, but also very expensive.
A common-mode filter without magnetic material is thus needed.
Further, in integrated technology, circuits are often needed to be protected against possible electrostatic discharges. A common-mode filter with an ESD protection is thus desireable.